alt Hacker NewsI will agree that the problem that class hierarchies attempt to solve is a problem one usually does not really have, but in your example you have not solved it at all.
It matches a relational database well, but once you have a just a user reference, you can not narrow it down to an employee without out of band information. If a user should be just one type that can be multiple things at once, you can give them a set of roles.
I agree that there are better ways to model roles and FGA
the point of the example wasn't to be an idiomatic solution to that problem, but to illustrate the pointlessness of inheritance in general, and User Student Employee was the first thing that came to mind that was more "real" than the usual Animal examples
in any case, as for only having a User reference, you don't ever have only a User reference - overwhelmingly you would usually load a Student or Employee,
and each of them would have those attributes on them that characterize each of them respectively (and those attributes would not be shared - if they were, such shared attributes would go on the User object)
only those functions that truly are operating only on User would you send in each of their User objects
the problem with what you're advocating is you end up with functions like this
which is insane